Conventionally, an active-matrix liquid crystal display device includes pixel sections independently arranged on a liquid crystal panel in a matrix form. In each pixel section, a pixel electrode and a switching element are provided.
In the active matrix liquid crystal display device, a driving voltage is applied to a pixel electrode via a switching element, and a liquid crystal is aligned based on a potential difference between the pixel electrode and a counter electrode provided facing the pixel electrode via the liquid crystal, so as to control ON/OFF of light transmitting therethrough, thereby displaying an image on the liquid crystal panel.
In the described liquid crystal display device, as a switching element, an MIM (Metal Insulator Metal) element or a TFT (Thin Film Transistor) element is used. Especially, a liquid crystal panel adopting TFT elements has the greatest demand in a variety of fields as an active matrix liquid crystal display device in view of quality and cost.
The liquid crystal display device adopting TFT elements includes scanning lines for inputting a signal for driving the TFT elements to pixel electrodes respectively formed in pixel sections arranged in a matrix form, and signal lines, formed so as to cross the scanning lines at right angle, for inputting a signal of an image to be displayed on the liquid crystal panel. The pixel electrodes are provided on the scanning lines and the signal lines via an insulating film.
The described liquid crystal display device having the arrangement wherein the pixel electrodes are provided on the scanning lines and the signal lines via the insulating film is disclosed in, for example, Japanese Unexamined Patent Publication No. 172685/1983 (Tokukaisho 58-172685). The arrangement for a liquid crystal display device as described in the above publication has the beneficial effect of improving the aperture ratio of each pixel and/or suppressing a misalignment of pixel electrodes by shielding an electric field generated from the signal lines by means of the insulating film.
Another liquid crystal display device is disclosed in Japanese Unexamined Patent Publication No. 242433/1994 (Tokukaihei 6-242433), wherein a black matrix pattern or a color filter layer are integrally formed on a substrate. This eliminates a need for considering whether an inferior alignment has occurred during the bonding of the substrate and the counter substrate together, thereby achieving a still further improved aperture ratio.
However, for the above-described case of providing the pixel electrodes on the scanning lines and the signal lines via the insulating film, the application of a source signal may cause the electrical charges on a charged electrode to move about due to an electrostatic capacitance created between the pixel electrodes and these lines (scanning lines and signal lines). This creates a phenomenon, known as longitudinal crosstalk, which becomes is most noticeable when displaying a black window pattern on a screen of an intermediate tone. In order to prevent this longitudinal crosstalk problem, it is necessary to adopt an insulating film material of electrical capacitance extremely small electric capacity for, i.e., a material of an extremely low dielectric constant. For this reason, an organic polymer thin film such as acrylic resin is used as the insulating film.
In addition, since switching elements such as TFT elements are likely to be adversely affected by electrical charges which generate a strong electric field, the static electricity generated during the manufacturing process of a liquid crystal display device may cause damage to the TFT elements. For example, in order to determine an alignment direction of liquid crystal material in a liquid crystal display (LCD) device), an alignment film made of polyimide, etc., is formed on the substrate. Then, the liquid crystal material in the vicinity of the substrate is aligned in one direction by rubbing the alignment film with a cloth, etc. In this manner, the static electricity produced by the rubbing generates an electric field which aligns the liquid crystal material. Consequently, when the scanning lines or the signal lines on the substrate are charged by the static electricity during this process, the crystalline structure of the the semiconductor layer in the TFT element is adversely affected. As a result, the threshold value of the gate voltage in the TFT element deviates. This hinders proper switching of the switching element, thereby causing defective operation of a pixel which is charged by the static electricity.
In order to prevent the above described problems, during the manufacturing process of the substrate, in general, all the input terminals of the scanning lines and the signal lines are short-circuited by a metal pattern, called a "short-ring". However, this short-ring is removed after the liquid crystal panel has been manufactured by bonding the substrate and the counter substrate together, and when mounting peripheral circuits to the input terminals by, for example, the TAB (Tape Automated Bonding) method. Therefore, the short-ring of the input terminals does not prevent the static electricity generated during the mounting process.
Thus, in order to prevent the described problem, an attempt is made to provide a protective circuit in the vicinity of the input terminals of the scanning lines and/or adjoining signal lines so as to connect the adjoining scanning lines and/or signal lines.
In general, as shown in FIG. 10, a liquid crystal panel is prepared by bonding an active matrix substrate 101 and a counter substrate 102 together by means of a seal material 103. On the active matrix substrate 101, scanning lines 106 and signal lines 107 are provided so as to cross each other at right angle. Each segment surrounded by the adjoining scanning lines 106 and the adjoining signal lines 107 serves as a pixel section 113. On both end portions of the scanning lines 106 and the signal lines 107 formed on the active matrix substrate 101, scanning line input terminals 108 and signal line input terminals 109 are formed respectively. Furthermore, on the scanning lines 106 and the signal lines 107, an interlayer insulating film 104 is formed. On the interlayer insulating film 104, the seal material 103 is applied so as not to overlap an effective display region 105. Counter electrode terminals 110 are formed parallel to the signal line input terminals 109. Each counter electrode terminal 110 is connected to a counter electrode (not shown) on the counter substrate 102 via a conductive material 111. A protective circuit 112 is provided so as to connect the adjoining scanning lines 106 and/or the adjoining signal lines 107.
As shown in FIG. 11, the protective circuit 112 can be formed using switching elements of a diode structure. The switching element is prepared by placing a gate electrode 115, a gate insulating film 116 and a semiconductor thin film 117 on the active matrix substrate 101 in this order.
In the source section of the semiconductor thin film 117, a source electrode 118a made of n.sup.+ -silicon layer is formed, and in the drain section, a drain electrode 118b made of n.sup.+ -silicon layer is formed. To the source electrode 118a, a metal layer 119a which serves as a source wiring is connected, and to the drain electrode 118b, a metal layer 119b which serves as a drain wiring is connected. Further, the interlayer insulating film 104 is formed so as to cover the switching elements entirely.
The protective circuit 112 may be formed, for example, as a diode ring structure wherein the described switching elements are connected in parallel and opposite directions may be adopted. The protective circuit 112 is provided so as to connect adjoining scanning electrodes 106 and/or adjoining signal electrodes 107. According to the described arrangement, even if an electric field of not less than a predetermined intensity is applied to one line 106 or 107, the charge can be released to the adjoining scanning lines 106 and/or the adjoining signal lines 107, thereby preventing an occurrence of an inferior liquid crystal display device as previously described.
However, in the process of connecting peripheral circuits to the input terminals of the liquid crystal panel, if foreign substances enter a bonded part by the TAB method, or the peripheral circuits are bonded to the input terminals in a displaced position, it is required to strip the part already bonded and mount the peripheral circuit again by the TAB method.
In order to strip the part already bonded from the input terminals, it is required to apply a large force. However, the insulating film made of an organic polymer thin film does not have a sufficient adhesiveness to the substrate or the gate insulating film, etc., unlike the metal thin film or the silicon thin film, etc. Therefore, depending on the strength of the force applied when removing the part already bonded from the input terminals, the insulating film under the bonded part may be stripped as well.
As the described protective circuit is provided in a vicinity of the input terminals of the scanning lines or the signal lines, the insulating film which covers the protective circuit may be stripped. If the insulating film which covers the protective circuit is stripped, as the protective circuit is exposed to the outside air, the semiconducting characteristics of the protective circuit deteriorate, and its performances suffer.
Furthermore, a distance between the source section and the drain section of the protective circuit is much smaller than the distance between the input terminals. Therefore, in the case where the protective circuit is exposed to the outside air, electrically conductive substances and humidity may adhere to the protective circuit, which causes a defect such as a leakage between terminals of the protective circuit. Such defect of the protective circuit adversely affects the display image of the liquid crystal display device.